Scale along face normal
I'm not entirely sure I'm clear on what your issue is. But any time you want to create a custom transform orientation, you can follow the instructions given by Carlo in the link to the Blender manual above easily enough. However, if you also want that custom transform orientation to be aligned with a face normal, you can press ⇧ ShiftNUMPAD7 to force your current camera view to instantly snap to a facing that views the currently selected polygon face head-on, allowing you to look straight down its normal's z-axis. Once this camera orientation is achieved, you can create a new custom transform orientation based on it. Then you can transform along that face's normal z axis, no matter how the object it belongs to is oriented in space.